1. Field of the Invention
This invention relates to a high strength and modulus film of conjugated polymer having excellent mechanical properties, a highly electroconductive carbonaceous material obtained by pyrolyzing a biaxially stretch-oriented film of the conjugated polymer, and a composition thereof.
2. Description of the Prior Art
In recent years, on the basis of the fundamental principle that by introducing a rigid molecular structure into the main chain of a polymer, there is formed a higher order structure which enable the polymer to exhibit its strength and elastic modulus to the maximum, attempts to produce a high strength and modulus film from a synthetic polymer material have increased rapidly.
Polyphenylenevinylene is a straight-chain conjugated polymer in which phenyl groups and vinylene groups are alternately aligned, has a rigid conjugated chain in its main chain, and hence is expected to be excellent in mechanical strength, but it is difficult to form into a film and its mechanical properties have hardly been investigated.
It is already disclosed that a polypehnylenevinylene structure can be formed by a method comprising subjecting a polyelectrolyte of sulfonium salt which is a soluble polymer intermediate to elimination of the sulfonium salt [J. Polym. Sci., A-1, 6, 1058(1968)](hereinafter referred to as "a sulfonium salt decomposition method"). Further, U.S. Pat. Nos. 3,532,643 (1970) and 3,706,677 (1972) disclose that a filmy polyphenylenevinylene molded article can be produced by employing the same reaction as described above, namely, molding a polyelectrolyte of sulfonium salt into a film by casting, followed by subjecting the resulting article to elimination of the sulfonium salt.
As to the mechanical strength of filmy molded article, it is merely described in the above U.S. Pat. No. 3,706,677 (1970) that the filmy polyphenylenevinylene molded article has a tensile strength of 4.9 kg/mm.sup.2 and an elongation percentage of 3.5%. These mechanical property values are not sufficient for an excellent film as compared with polyimide films or stretch-oriented polyester films which are known as high strength and modulus films, and mechanical property values sufficient as those aimed at by this invention have not been able to be attained by merely making a simple application of the method disclosed in this patent.
The present inventors noted that a homogeneous polymer film having a phenylenevinylene structure and comprising the conjugated chain thereof as the main constituent could be produced by the sulfonium salt decomposition method, and devoted themselves to research on a method for improving the mechanical properties of said film. As a result, the inventors have found that there can be obtained a high strength and modulus film which have greatly improved performance characteristics and satisfy the condition of mechanical properties above a definite level, whereby the inventors have accomplished this invention.
The present inventors considered that the cause of the low mechanical strength of a polyphenylenevinylene film obtained by the aforesaid well-known sulfonium salt decomposition method is an occurrence of oxidation in the course of the elimination of sulfonium salt, which made improvement of the mechanical properties impossible. Therefore, the inventors have extensively investigated various synthesis conditions for producing a high strength and modulus film, and have consequently found that control of the atmosphere at the time of the elimination of sulfonium salt and stretch-orientation are very important for improving the mechanical properties greatly.
The inventors have also found that a stretch-oriented film having a high strength and modulus can be obtained not only by subjecting a p-phenylenevinylene polymer to stretch-orientation but also by subjecting thereto a conjugated polymer comprising, as recurring unit, vinylene group and aromatic hydrocarbon group conjugated therewith, whereby the inventors have accomplished this invention.
Further, the present inventors noted the molecular structure and higher order structure of the stretched film, especially, biaxially stretch-oriented film of conjugated polymer, and investigated the pyrolysis conditions of the polymer film at a higher temperature in more detail. As a result, the inventors have found that the pyrolyzed product of the biaxially stretch-oriented film becomes a very excellent highly electroconductive carbonaceous material.
The present inventors have already found that a film of p-phenylenevinylene polymer, which is a conjugated polymer, not only can be carbonized while retaining its film form without fusion even when pyrolyzed at a temperature above 400.degree. C., but also undergoes progress of graphitization at high temperatures to become a highly electroconductive carbonaceous material, and that said film is given a higher electroconductivity by doping, and the inventors have proposed it in U.S. patent application Ser. No. 622,582, now U.S. Pat. No. 4,599,193.
The present inventors investigated a method for further improving the electroconductivity, and have consequently found that when a biaxially stretch-oriented film of conjugated polymer, for example, a phenylenevinylene polymer is pyrolyzed in an inert atmosphere at a temperature above 400.degree. C., it not only can be carbonized while retaining its form without fusion but also is readily graphitized by pyrolysis at a high temperature to become a carbonaceous material having a higher electroconductivity than a similarly pyrolyzed unstretched or uniaxially stretch-oriented film, the electroconductivity of which material reaches 10.sup.4 S/cm, i.e., a value near that of the electroconductivity of graphite single crystals and becomes as higher as 10.sup.5 S/cm by doping.
That is to say, the inventors have found that in pyrolysis of a biaxially stretch-oriented film, the electroconductivity of pyrolyzed film is improved with an increase of the biaxial-stretching ratio before the pyrolysis.
In the case of a uniaxially stretch-oriented film, the electroconductivity after pyrolysis was insufficiently improved with an increase of the stretching ratio, and even excess stretching had a slight improving effect on the electroconductivity.
It was a surprise that thus, biaxially stretch-oriented poly-p-phenylenevinylene could be made into a carbonaceous formed article by pyrolysis without fusion or softening and moreover could be made into a carbonaceous material having a marvolous very high electroconductivity as compared with an unstretched or uniaxially stretch-oriented film. It could not be expected that biaxial stretch-orientation was very effective. It is also very characteristic that a highly electroconductive carbonaceous material can be obtained stably with good reproducibility by using a biaxially stretch-oriented film for pyrolysis.
The fact found here that a highly electroconductive material can be obtained by virtue of biaxial stretch-orientation was found to be the case not only with pyrolysis of poly-p-phenylenevinylene but also with pyrolysis of a conjugated polymer comprising, as recurring unit, vinylene group and aromatic hydrocarbon group conjugated therewith after biaxial stretch-orientation.